Temperature evolution
Temperature evolution in different parts of the world emerging through reconstructed thirty-year averages. Colours indicate relative temperature. For North America, there is both a shorter reconstruction based on tree rings and a longer reconstruction based on pollen. In Africa, there is not yet enough data to make any temperature reconstructions.

The researchers, coordinated by the Past Global Changes (PAGES) project, reached their conclusions by integrating 511 series of historic climate data from tree rings, ice cores, pollen, corals, lake sediments, and other data. Similar to what previous studies have shown, the new research confirms an overall cooling trend across nearly all continents over the last 2,000 years. The cooling was likely caused by changes in Earth’s orbit, but the global warming during the last century seems to have reversed this long-term cooling. Two researchers from Stockholm University, Fredrik Charpentier Ljungqvist and Paul J. Krusic, were involved in the project.  

“This is the first time that it has been possible to compare temperature trends between different continents in this way. It is interesting to note that the Medieval Warm Period starts around 200 years later in the southern hemisphere as compared to the northern hemisphere, and the Little Ice Age also starts later in the south,” says Fredrik Charpentier Ljungqvist.

Differences between continents during the Medieval Warm Period (c. 800-1300) are larger than during the Little Ice Age (c.1300-1900). Direct comparisons between past and present climates are difficult, but the study indicates that the temperature of the last 30 years has possibly been higher than during any other period in the last 1-2,000 years on several continents.

“We can see that it was warm in the northern hemisphere in the period between 830 and 1100, whereas the warm period lasted between 1160 and 1370 in the southern hemisphere. Between 1580 and 1880, however, there was a cold climate nearly everywhere in the world,” says Fredrik Charpentier Ljungqvist.

Researchers have found that the coldest periods, especially when they coincide in different continents, occured when there was a decrease in solar activity and an increase in volcanic activity at the same time. Five periods between 1250 and 1820, i.e. during the Little Ice Age, were particularly cold, but not always on all continents simultaneously.

“The only continent that has bucked the modern warming is Antarctica. In northern Europe, it was probably at least as warm in the year 1000 as it is now, but in southern Europe, the past decade has probably been the warmest since Roman times. That shows how important it is to study regional variations in order to understand climate change,” says Fredrik Charpentier Ljungqvist.

Reference: PAGES 2k Consortium, “Continental-scale temperature variability during the past two millennia”, Nature Geoscience 6 (2013): 339–346

Frequently asked questions about this article, other resources, and additional contact information are available at: http://www.pages-igbp.org/workinggroups/2k-network/faq

For further information
Fredrik Charpentier Ljungqvist, Department of History, Stockholm University, tel: +46(0) 70 662 07 28, email: fredrik.c.l@historia.su.se